Measuring instrument



Dec; 23, '1952' v. DURBIN E I'AL 2,623,093

MEASURING INSTRUMENT Filed-Nov. so. 1948 Patented Dec. 23, 1952 UNIT PATENT ()FF'IGE 2,623,098 ME AsfiRrN G manor/rem Vernon"Dui'-bii1,'Waban, anaftirnst vvljonnsnn;; Eirer'ett;Mass.;assignors to National Pneumatic, Co. Inc.,- Boston',-Mass., a corporation" oi Delarsisnatiaanaeaita e0, 194's; Serial N61" 62,622

The present invention ref tests rneasuringinstruments" and more; specifically 'to instruments for the measurement'of electr' arresistanea ar; ticularly Where high resistances oi the: order of one megohm or" greater arinvo vea; as testing insulation. I I

Direct reading instrur'iie ment of resistances, whi for portable use and in a bar of readings mustbe tak ject to the disadvantage that a source of poten} tial must be provided which is; substantially con stant and of known vane; m the cas'ofinst'ruments for themeasureiiient of resista ces' alcove one million ohms, the"po'tentia1" aya lemiis't be of the "order of sevr'armiii red volts ifthe indicating" instrument to malieuse of a' conventional meter indve'ment,siich'as is common in milliamm'eters. v

To supply the' required high Iioteiitia1 for a conveniently portable high resistance" meter or megohm-meter, sinall hand-cranked generators of the rotating field type have been; devised. When used inconjunctionwith atfsuitiihi rectifier and sirnple filtrya'relatively" purejfD'. C output may be obtained. The readinobta ined, how ever, varies with the speed at which-fthefgen erator is cranked by the operator wh-ileareading is being taken. V j I 7 To avoidthenees'sity of'cr'ankin the generato'r at soms eedetermi ned uniforrfi'speed in order to obtain 'a known and'c'onstant vol tage f or use in carrying out the j resistance measurement,

the present invention has as" an object the provi sionin a compact, portable"meg6hmm'eter em" pioying a hand cranked generation-of means for stabilizing the resistance reading-when the generatorisc'ranked at a rateiri efxces's' of'a minimum rate and for indicating "automatically"to theoperator when the"crankin gisproceeding'at a rate which results in stabilizingtheireading.

in'carrying out 'the invention, use-ism'ade of the regulatin'g effect of a 'lig lit'd ne'cma np or similar type" of glow disch'ai'ge"device} whereby a substantially constant potential drop acro s the meter is obtained" over a"'r1ative1y widrange of generated voltages? Inthe preferre construction, several "such lamps are-connected in series. It is only'necsa'ry thatthe voltageo'f tiief'sm'n cfe be maintained 'above'the minimum value which is vention contemplates that the megohn'i ineter' be so constructed and" arrangdthat the 'coii'di tion of the neon lamps will be visible to' the as:

2 era'tor in order to indicate whether the" apparatus is being actuated pronerly" for securing accurate readings; By maintaining the cranking speed at a rate whiohfause's the lainp's tdglow} 'the o pf erator is" assured"thatthecranlring'speed is sure: cient'to maintain the neoir'iampfsmccntroi of the voltage across'thei meter terminals and thus permit accurate measur'ments" of resistance to be'ma'de; Y H v Inth drawings illustrating the invention, Fig? ure' 1 is a view in s'id' awaits; partly broken away, showing a'self-ont'aind resistance'mea's uring-iand' insulation t'stin'g instrument embody ing the invention, and" Figurez is 'a"wi r'in;g di' agram 'of the instrument; illustrating'the'various components thereof in conventional schematic fashion. a, a v v The highpotential'nc'ssary for direct-reading measurement of resi'stan'ces of one ih'egohm or higher is provided" by a smallhand cranked gen erator 8' having a rotating peri'naiient magnet field" HI. Speedmu'ltiplying'connections of stand} ard' arrangementpermit therotonto be turned at high speediby operating" thesmali' crank" l2 provid'd'for thvpurposeL Th'ge'nerator is provided with a single continuous windingfin two sections" as" shown) from whichma'y" he derived an alternating'current' voltage" of suificient' in tensity for the operationtohe described. I The output of" the" generator is converted to direct current by 'means 'of a rectifier l9, which may be of the cold" cathode" type'having' a main anode and'catho'de and an auxiliaryor' starting" anode in circuit with suitable high resistance units 2|" so; thatthis"auxi1iary' or starting circuit wili function as a means of initiating the? cycle ofoperation; after which the'rectifyi'ngacticn'will proceed byway 'of the main anode cathode c'ircuit"inclei e*ndent1y' of the" high resistance shunt:

snicctnedty a simifle capacitantfiltert'z which; requ r'esno limiting resistance 'unit' when used voltage: may he appliedtd'the actual resistance as ubs'tantial potential drop between p'oi cranked above a" certain minimum rate. The

3 in series between points A and B, and hence across the output of the generator and filter.

The glow discharge lamp has the property of lighting, and thereby passing current, when the applied potential exceeds a potential known as the striking potential. Provided that the D. C. potential'exceeds the striking potential, the lamp will remain lighted, but if the potential applied drops much below the striking potential, the lamp will extinguish itself. As long as the lamp remains lighted, the potential drop across the terminals remains adequate for stabilization of the reading on meter 28 regardless of variations in the supply voltage. In the type of device found most useful as a portable testing unit, the preferred voltage between A and B is in the neighborhood of 500 volts. This voltage is most conveniently obtained by the use of eight lamps in series, each having a potential drop of approximately 70 volts.

It should be noted that this potential (500) volts is not that impressed across the unknown resistance, but to the measuring circuit composed of the meter 28 with its calibrated shunt 34, the unknown resistance (connected across the meter terminals 36) and the fixed resistance 30 which functions as a protective resistor to limit the meter to full scale deflection (indicating zero resistance) in case the external or insulation resistance under test proves to be a short circuit. Because of the drop across the meter and across the fixed resistor, the potential applied across the unknown resistance will be less than the drop between A and B. However, with a suitable designed instrument, even in the case of insulation or other resistance under test having a resistance as low as one megohm, the potential applied across the same will be in excess of three hundred volts.

As has already been pointed out, the condition of stabilization of the resistance measuring circuit is contingent upon cranking the generator at sufficient speed to maintain the lamps lighted. In order that the operator may be assisted in maintaining the cranking speed at a rate which is at least equal to the minimum speed which will maintain the condition of adequate potential drop across the lamps, means are provided to enable the operator to observe the condition of the lamps while cranking. As long as the lamps are lighted, the operator is assured that the cranking speed is adequate and that resistance measurements may be made with accuracy.

In the embodiment illustrated in Figure '1, the several neon-type glow discharge lamps are arranged in closely spaced relation on a mounting strip 40 extending transversely of the instrument case. In order that their condition of illumina tion may be observable from outside the case and over a wide range of angles, light transmitting means in the form of rods 42 of a transparent plastic material having a high index of refraction, for example a material such as that commonly identified as Lucite, are mounted in alignment with one or more of the lamps. The outer ends of these rods are rounded so as to project slightly above the surface of the case, and said rounded surfaces 44 are frosted or ground to have a matte surface. As a result, these frosted ends are caused to glow when the lamps within the case are lighted, and the glow may be observed from all directions in which the rod end is visible. Due to the series connection of the neon lamps, it is necessary to observe the condition of but one lamp to determine whether the lamps are in control of potential, since any one lamp can become lighted only by the establishment of current flow and consequent lighting of all the other lamps in the chain. For the convenience of the operator, however, more than one rod may be used if desired, for example a rod may be used at each end of the lamp bank to provide indications at spaced points on the outside of the case.

The present invention thus makes possible a compact resistance measuring and insulation testing instrument wherein the hand-cranked source of potential is automatically stabilized to permit accurate measurements as long as the generator is cranked above a certain critical speed, a continuous check on the cranking speed being proved by observing the condition of lighttransmitting means in optical communication with the voltage-stabilizing neon lamps within the case. As long as the neon lamps are lighted, as shown by observation of the indicating means, the cranking speed is satisfactory for taking resistance measurements or testing insulation.

It occasionally happens that there is sufficient condensation to form a leakage pathof high resistance, of course-between the terminals 36, 36 and the instrument housing. Such a leakage path, even though of high resistance, would interfere with the accuracy of the meter reading. To compensate for this there is provided a guard circuit 46 including a grounded high resistance 41, to serve as a shunt path around the meter on such occasions, and thus reduce any error due to such leakage paths.

The insulation under tests, as indicated at 35 in Fig. 2, is merely illustrative of the manner of utilizing the invention. The same principles disclosed may obviously be applied to the measurement of resistances other than wire insulationfor example, the resistance between two conductors, or between a conductor and any other metallic part.

This application contains subject matter originally presented in ourapplication Ser. No. 518,723, filed January 18, .1944, and subsequently abandoned. I.

. What we claim is: I

1. In indicating apparatus, a circuit including a test section whose resistance is to be measured, a manually rotatable current generator, a meter including a scale on which the resistance of said test section is directly readable, a lamp circuit adjacent said meter, said lamp circuit being connected directly across the terminals of said generator and therefore subject to generator potential, said lamp circuit including gaseous type glow discharge lamps one of which serves as a check lamp, means for illuminating said check lamp and thereby indicating that the meter operation has become stabilized, and means including all other lamps of said lamp circuit for delaying operation of said illuminating means until said generator reaches a pre-determined speed.

2. Apparatus as defined in claim 1, wherein said delaying means further includes an electronic device having a pair of electrodes across which generated current flows as soon as the generator begins to rotate, and a third electrode to which the generated current is diverted in response to such initial flow.

3. Apparatus as defined in claim 2, wherein said generator includes a pair of field windings in series with said electronic device, and with each other.

ammo

4 tion.

5. Apparqtus as defined in claim 2, wherein said generator windings inherently provide sufll cient resistance for control of the rectifying action of said velectronic device.

VERNON DURBIN. ERNEST W. JOHNSON.

REFERENCES CITED The following references are of record in the file of this pit-tent:

Number 8 UNITED STATES PATENTS Name Date Evershed June 25, 1907 Rolfe Aug. 21, 1928 Crago July 18, 1933 Rolfe Oct. 21, 1941 Delmhorst Feb. 10, 1942 Trucksess Aug. 10, 1943 Moore Oct. 28, 1947 Love Jan. 25, 1949 

